1. Field of the Invention
The present invention generally relates to integrated circuits usable as driver ICs for driving liquid-crystal-display panels, and particularly relates to an LCD data driver that drives data-bus lines of a liquid-crystal-display panel based on display data.
2. Description of the Related Art
A liquid-crystal-display panel has pixels comprised of transistors arranged in a matrix form, with gate-bus lines extending in a horizontal direction connected to the gates of the pixel transistors, and data-bus lines extending in a vertical direction connected to pixel condensers via the transistors. When data is to be displayed on the liquid-crystal-display panel, gate drivers successively drive the gate-bus lines one after another so as to make transistors conductive on a selected horizontal line. Data drivers write data in pixels on the selected horizontal line through the transistors that are made conductive.
In a conventional configuration, generally, the LCD data drivers are commonly connected to a bus for transferring display data signals, clock signals, etc. In such a configuration, signal lines intersect with each other, resulting in a large number of layers provided in an implemented substrate. In order to reduce the number of substrate layers, the LCD data drivers may be connected in cascade connection, thereby supplying the outputs of a given LCD data driver to a next LCD data driver provided at a subsequent stage.
The cascade-connection configuration can reduce the number of substrate layers because the LCD drivers are connected in series without having intersections between the implemented signal lines. This provides a basis for manufacturing the substrates at low costs.
With the LCD data drivers arranged in cascade connection, inputting of a signal into a given driver device results in this signal being supplied to a next driver device via an output buffer. Since a positive transition and a negative transition of a signal have different delays in the buffer due to variation in the manufacturing process, the output signal may end up having a slightly different duty ratio than the input signal.
When LCD data drivers having similar delay characteristics are connected in cascade connection, an error of the duty ratio will be accumulated each time the signal passes through one of the LCD data drivers. After passing through a substantial number of drivers, the error of the duty ratio reaches to such a level that the error cannot be ignored. In an LCD panel of an SXGA type, for example, 10 LCD data drivers are connected in cascade connection, so that accumulated errors of a duty ratio may result in a signal failing to properly propagate.
Accordingly, there is a need for an LCD data driver which is free from accumulation of duty ratio errors, and, also, there is a need for a liquid-crystal-display device using such LCD data drivers.